The combustion reaction

Consider the combustion of wood.

The chemical nature of wood is closely related to sugars. To make things simpler,
let's consider wood to be composed just of Sugar,
whose formula is C6H12O6

Actually, wood is composed mainly of Cellulose, that is a polymer made up by repetition of Glucose residues. Glucose
is a sugar, and cellulose formula is C6H10O5

When burning, wood reacts with Oxygen, which is contained in air:

C6H12O6

+

6 O2

=

6 CO2

+

6 H2O

The two reaction products are Carbon dioxide CO2 and Water H2O.
Both are released as gases in air. Carbon dioxide is a normal component
of air; water is generally known as a liquid, but it is a gas at the
high temperature of flame. Indeed, gaseous water is a normal component
of air (atmospheric humidity).

So in wood combustion we observe solid wood disappear and be
converted to gas products, leaving only some ashes. These are formed by
minor components of wood that cannot burn and remain solid.

The other major effect of combustion is production of heat and light. See the page on energy balance for a discussion.

A chemical reaction is a transformation where some molecules are
destroyed and new molecules are formed. In the above example, sugar and
oxygen molecules disappear and water and carbon dioxyde molecules
appear. Molecules are not conserved across chemical reactions.

The product molecules, however, are formed by exactly the same atoms that made up the reactant molecules. Atoms are conserved across chemical reactions, but are rearranged in different molecules.